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+# Qwen27B Chat 0-8k Harness Ablation
+
+Date: 2026-05-10
+
+## Setup
+
+- Host: `dash0` (`172.27.114.84`)
+- Model: `/home/admin/resource/model/464482ce/qwen3.5-27b/256k-0223-internal`
+- Workload: chat, 0-8k input window
+- SLO: TTFT <= 4000ms and TPOT <= 25ms, target pass rate = 0.95
+- Trial budget: 12 total tuning iterations per study
+- Execution: direct `python3 -m aituner.cli study tune ... --max-trials 12`
+- GPU env: `CUDA_VISIBLE_DEVICES=0,1,2,4,5,6,7`
+- Code commit: `adc4351`
+
+The previous no-harness run was affected by the `dash0` migration and had many engine launch failures. This document uses the clean no-harness rerun from 2026-05-09.
+
+## Studies
+
+| Variant | Study ID |
+| --- | --- |
+| no-harness rerun | `dash0-qwen27b-chat-0-8k-ttft4s-tpot25-gpu3skip-12iter-noharness-rerun-20260509` |
+| harness | `dash0-qwen27b-chat-0-8k-ttft4s-tpot25-gpu3skip-12iter-harness-20260508` |
+
+## Result
+
+| Variant | Best iter | Best request rate | Best request rate / GPU | Best config summary |
+| --- | ---: | ---: | ---: | --- |
+| no-harness rerun | 10 | 0.4050 | 0.2025 | `tensor-parallel-size=2`, `data-parallel-size=1`, `max-num-batched-tokens=12288` |
+| harness | 8 | 1.0967 | 0.2742 | `tensor-parallel-size=4`, `enable-chunked-prefill=true`, `max-num-batched-tokens=16384` |
+
+Harness reached a higher incumbent and did so earlier. Final best request rate per GPU improved by about `35.4%` over the clean no-harness rerun.
+
+## Incumbent Curve
+
+Values are incumbent best request rate per GPU after each tuning iteration.
+
+| Variant | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
+| --- | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: |
+| no-harness rerun | 0.0650 | 0.0650 | 0.0650 | 0.0650 | 0.0650 | 0.0650 | 0.0650 | 0.0650 | 0.0650 | 0.2025 | 0.2025 | 0.2025 |
+| harness | 0.0650 | 0.0650 | 0.2025 | 0.2025 | 0.2025 | 0.2025 | 0.2696 | 0.2742 | 0.2742 | 0.2742 | 0.2742 | stop |
+
+## Trial Details
+
+No-harness rerun:
+
+| Iter | Trial result / GPU | Incumbent / GPU | Status | Config summary |
+| ---: | ---: | ---: | --- | --- |
+| 1 | 0.0650 | 0.0650 | completed | baseline |
+| 2 | 0.0617 | 0.0650 | completed | `tp=1`, `dp=2`, `max-num-batched-tokens=12288` |
+| 3 | 0.0308 | 0.0650 | completed | `tp=1`, `dp=4` |
+| 4 | - | 0.0650 | completed, infeasible | `max-num-batched-tokens=12288` |
+| 5 | - | 0.0650 | completed, infeasible | `tp=1`, `dp=2`, `max-num-batched-tokens=16384` |
+| 6 | - | 0.0650 | completed, infeasible | `tp=1`, `dp=2`, `max-num-batched-tokens=12288`, `block-size=32` |
+| 7 | - | 0.0650 | completed, infeasible | `max-num-batched-tokens=10240` |
+| 8 | - | 0.0650 | completed, infeasible | `max-num-batched-tokens=7168` |
+| 9 | - | 0.0650 | completed, infeasible | `tp=1`, `dp=2` |
+| 10 | 0.2025 | 0.2025 | completed | `tp=2`, `dp=1`, `max-num-batched-tokens=12288` |
+| 11 | - | 0.2025 | completed, infeasible | `tp=2`, `dp=1`, `max-num-batched-tokens=10240` |
+| 12 | - | 0.2025 | completed, infeasible | `tp=2`, `dp=1`, `max-num-batched-tokens=13312` |
+
+Harness:
+
+| Iter | Trial result / GPU | Incumbent / GPU | Status | Config summary |
+| ---: | ---: | ---: | --- | --- |
+| 1 | 0.0650 | 0.0650 | completed | baseline |
+| 2 | 0.0617 | 0.0650 | completed | `tp=1`, `dp=2` |
+| 3 | 0.2025 | 0.2025 | completed | `tp=2`, `dp=1` |
+| 4 | - | 0.2025 | completed, infeasible | `tp=2`, chunked prefill, `max-num-batched-tokens=16384` |
+| 5 | 0.1283 | 0.2025 | completed | `tp=2`, `dp=2` |
+| 6 | - | 0.2025 | completed, infeasible | `tp=2`, `dp=1`, `max-num-seqs=4` |
+| 7 | 0.2696 | 0.2696 | completed | `tp=4`, `dp=1` |
+| 8 | 0.2742 | 0.2742 | completed | `tp=4`, chunked prefill, `max-num-batched-tokens=16384` |
+| 9 | - | 0.2742 | completed, infeasible | `tp=4`, chunked prefill, `max-num-batched-tokens=24576` |
+| 10 | - | 0.2742 | completed, infeasible | `tp=4`, chunked prefill, `max-num-batched-tokens=16384`, `max-num-seqs=8` |
+| 11 | - | 0.2742 | completed, infeasible | `tp=4`, chunked prefill, `max-num-batched-tokens=16384`, `max-num-seqs=16` |
+| 12 | - | 0.2742 | harness stop | validation exhausted after strong incumbent |
+
+## Interpretation
+
+The clean no-harness rerun eventually found the `tp=2` topology at iter 10, so the old migration-tainted no-harness result was indeed too pessimistic. Harness still improves the process in two ways:
+
+- It reaches the `tp=2` topology by iter 3 instead of iter 10.
+- It then escalates to `tp=4` and a nearby batching refinement, reaching `0.2742 req/s/GPU`.
+
+The harness effect is not "one iter to best"; it is directional search. It turns bottleneck evidence into topology validation probes, then validates runtime refinements around the stronger incumbent and stops when further nearby probes do not improve.